Resolution in isotachophoresis or capillary zone electrophoresis is controlled by the diameter of the separation channel. Resolution changes with the square of the ratio of an initial diameter a second (i.e., changed) to diameter of the separation channel. Therefore, as small an operating diameter as possible is sought. The design and construction of the detectors for such systems provide the practical limit for reduction of the diameter of the separation channel. Commercially available capillary electrophoretic instruments use 0.3 mm to 0.5 mm diameter capillaries, but Everaerts, Beckers and Verheggen (Isotachophoresis, Elsevier Scientific Publ. Co., Amsterdam, 1976, p. 395) describe the construction of an instrument embodying a 0.2 mm diameter separation channel (i.e., a 2.25 to 6-fold improvement in resolution). These authors also describe the construction of the detectors for such a system, detectors which require difficult and mechanically challenging construction techniques.
The detectors of choice for modern capillary electrophoretic analysis separations are the conductivity detector and the UV absorbance detector. These detectors should be mounted directly in the separation channel in order to retain the high resolution obtained by use of very small separation channels (0.2 mm in diameter or smaller). This places stringent demands on the introduction of light transverse to the separation channel for the UV absorbance detector. The large voltage gradient utilized for isotachophoresis or capillary zone electrophoresis also places severe restrictions on the width of the electrodes in contact with the separation channel in the conductivity detector. Thus, for a 15000 V gradient applied across a 20 cm long separation channel of 0.2 mm ID, the electrodes must be less than 10 .mu.m thick; otherwise electrolysis occurs at the extreme edges of each electrode. For the same reason electrodes must also be precisely perpendicular to the axis of the separation channel.
It is a primary object of the present invention to provide an improved means of constructing the conductivity and optical detectors for use in small bore isotachophoresis or capillary electrophoresis. One prior art technique for making a simple conductivity detector employs hot platinum wires melted through the wall of the polytetrafluoroethylene (PTFE) capillary used for the isotachophoretic separation channel. This is described by Kaniansky, et al. 267 Journal of Chromatography 67 (1983).
It is total volume of the separation channel which is important in an isotachophoretic separation rather than the length of the channel. Accordingly, volume coupling may be employed as disclosed by Verheggen and Everaerts, 249 Journal of Chromatography 221 (1982). The construction of a volume coupling system as described by Verheggen and Everaerts requires several interconnections between capillaries of different size, with the attendant problems of alignment and sealing.
Another object of the present invention is to provide a method for easily producing volume coupling in combination with the detectors and separation channel described. Other objects, features, and advantages will be apparent from the following description and appended claims.